1. Field of the Invention
The present invention relates to a back off sub for use in oil and gas drilling.
More particularly, the present invention relates to a back off sub for use in oil and gas drilling including a control section, an intermediate section and an abandonment section, where the intermediate section includes a connection that when broken, separates the abandonment section from the intermediate and control sections.
2. Description of the Related Art
While drilling oil wells, particularly highly deviated well-bores, the drill string can get stuck due to well-bore instability, failure to clean the hole adequately, very permeable low pressure zones, etc. When a drill string gets stuck, much effort is expended in getting it unstuck by pulling, slacking off, torqueing and jarring on it. If these efforts fail to unstick the drill string, it becomes necessary to get the portion of the drill string that is above the stuck point separated or disconnected from the lower, stuck portion of the string.
Current art calls for rigging up a wireline unit, locating the free-point then using an explosive charge in combination with applied left hand torque to cause a threaded “tool joint” to unscrew. Often the first attempt does not successfully cause a tool joint to unscrew and the job must be repeated. If the string does not unscrew after several attempts, a larger explosive charge is run into the string on wire-line that will destroy a tool joint, thus freeing the unstuck portion of the drill string from the stuck portion.
The purpose of the invention disclosed herein is to allow a drilling string used in an oil well to be unscrewed or backed off in the event it becomes stuck while drilling an oil or gas well.
An oil well is drilled with a drill string. A drill string is a length of individual joints of tubing, connected by threaded joints called tool joints. The string extends from the surface to the bottom of the hole where a drill bit is connected to the bottom of the string. Usually, the entire drill string is rotated in a clockwise manner when viewed from the top transmitting torque from the surface to the bit to enable the bit to drill rock at the bottom of the hole. In addition to transmitting torque, the tubes are hollow and fluid called “mud” is pumped down the tubing and out the bit to cool the bit, assist the bit in cutting the rock and to lift the rock cuttings back to the surface. The string has to have enough tensional capacity for the upper portion to support the entire string weight. Torque requirements typically vary from 2,000 ft-lbs to 30,000 ft-lbs. Pressures typically vary from 2,000 psi to 7,500 psi and tension can vary from 50,000 lbs to more than 1,000,000 lbs.
Tool Joint
Tool joints, the threaded connectors that allow the drill string to be connected, generally have a larger outside diameter that the tubes and are typically 1-2 feet long. The female portion of the tool joint is welded to the upper portion of the tube and the male portion of the tool joint is welded to the lower portion of the tube. Drill string tool joints are typically screwed together with very high make-up torque. 6⅝″ Full Hole connections can be made up to 56,000 ft-lbs of torque. A similar amount of torque is normally required to unscrew the connection.
Tool joints are designed with metal flats that allow the connection to be preloaded to provide higher strength and stiffness and also to provide a metal to metal hydraulic seal. When a connection is broken, a large amount of torque is required to start the un-screwing rotation but once started, further rotation requires little torque. When the connection is screwed together, very little torque is required until the metal faces come together and then very high torque is required for the last small amount of make-up.
Drill String Composition Including Jars
A drill string is typically made up of 2 sections, the uppermost, extending from the surface to within a few hundred feet of the bottom, is drill pipe. Drill pipe typically has tube OD's of 3½″, 4½″, 5″, 5½″, 5⅞″ and 6⅝″ and are usually 28-30 ft long. Tool joints are welded to both ends of the tube with the upper tool joint having a female threaded connection and the lower tool joint having to male threaded connection. A joint of drill pipe with tool joints is usually around 31 ft long. At the bottom of the string, there is a series of specialty type tubular joints called the BHA. This section of the string usually consists of 2 additional types of tubulars, heavy weight drill pipe and drill collars. A BHA's main purpose is to provide weight to place on the bit and to house electronic downhole instruments. The drill collars typically have the largest outside diameters in the string and the heavyweight drill pipe is made up of tubes similar in OD to the drill pipe but with smaller inside diameters, therefore weighing more than the normal drill pipe. Most drilling BHA's have tools called “jars” in them which can often free a BHA if it becomes stuck. Jars are tools that can induce large impact forces to the string by either pulling or resting large amounts of weight on them. They cause impact by mechanically or hydraulically storing large amounts of tension and/or compression energy and then releasing it suddenly. Jars are normally located in the heavyweight drill pipe but can also be located within the uppermost section of the drill collars.
How a Drill String can Get Stuck
When drilling with a drill string, a new hole is opened up by the bit, allowing the drill string to progress downward. Occasionally, the hole can collapse, usually in a new hole, around the largest components of the BHA, the drill collar, sticking the drill string within the hole. Drill strings can also become differentially stuck which is caused by a hydraulic imbalance between the drill sting and low pressure, permeable sections in the hole. The drill string is stuck if it cannot be removed from the wellbore with normal or even elevated surface tension on the string. Often, the annular area around the drill collars can sometimes collapse enough to block the passage of mud from the bit to the surface. When a sticking event happens, the initial method used to free the string is to attempt to “fire” the jars. If the string is stuck below the jars, they can be fired by alternately pulling large amounts of tension on the string and waiting for the jars to fire upwards or lowering large amounts of weight on jars and allowing them to fire downwards. Operators typically jar on the stuck portion of the string until the drill string is free or until the jars quit working. If the string becomes stuck above the jars, there is no method to free the string from the surface other than pulling, relaxing and twisting.
If the drill string cannot be freed, the drill string must be disconnected above the stuck point.
How Wire-Lines are Used to Free Stuck Drill Strings
Currently, the normal method used to disconnect the unstuck portion of the string from the stuck portion is to use wire-line equipment. As soon as it appears that the string cannot be jarred free, a wire-line “back-off” service company is mobilized to the drill site. Depending on the location of the rig (offshore, on land, in a remote location, etc.), it can take from a few hours to days to mobilize wire-line back-off equipment.
Once the equipment and personnel are at the drill site, the wire-line equipment is rigged up and special tools and explosives are run down the center of the drill string tubes. A back-off explosive is typically made of up to 600 grams of primer cord to back-off a large drill string connection. The special tools can locate tool joints and also contain strain gages that are used to determine where the pipe is free. The wire-line tools are periodically set in the inside of the drill string and the drill string is pulled and twisted. The wire-line tool measures the strain of the drill pipe both in torsion and in tension and can determine if the drill string is free or stuck at the point the measurement is taken. Once the point at which the drill string is stuck is determined, tension in the string is adjusted to allow the point at which the string is to be backed off to be neutral and left hand torque is worked down the drill string from the surface. The amount of left hand torque worked into the string has to be lower than the torque required to unscrew a connection so as not to unscrew the drill string at a point above the desired one. When an adequate amount of torque has been worked into the string, the explosive charge, which has been positioned inside the tool joint that will be unscrewed, is detonated. The detonation acts as a large impactor that allows the joint to unscrew with significantly less left hand torque than would normally be required. Very often, it requires more than one detonation to unscrew the joint. If, after several attempts, the tool joint still does not unscrew, a larger explosive charge is run on wire-line that can cause the female portion of the tool joint to split and enlarge allowing the pipe to become separated without any left hand rotation.
Major Disadvantages
The major disadvantage to the use of wire-line to back-off a stuck drill string is the time required to mobilize, rig up and deploy wire-line tools, equipment and personnel. It often takes 12-24 hours to just be in a position to attempt a back-off. Hole conditions typically deteriorate with time and the point at which the string is stuck can move up rapidly, often sticking even more of the string.
Thus, there is a need in the art for an improved apparatus and method for back-off a struck section of drill string to decrease down time and to facilitate down hole operations.